A range of strategies have been proposed to enhance muscle bulk and strength as a treatment for a number of age-related muscle disorders and various neuromuscular disorders, including muscular dystrophies. Myostatin, a transforming growth factor-β family member, also called growth and differentiation factor-8, is a negative regulator of muscle growth and the myostatin signalling axis has been a major focus in such strategies. Myostatin null or hypomorphic animals are significantly larger than wild-type animals and show a large increase in skeletal muscle mass.1 The first natural myostatin mutation in humans has also been identified in a young boy.2 Myostatin blockade, therefore, offers a strategy for counteracting muscle-wasting conditions including Duchenne muscular dystrophy.3 Delivery of myostatin-inhibiting genes, including growth and differentiation factor-associated serum protein-1 (GASP-1), follistatin-related gene (FLRG), follistatin-344 (FS) and myostatin propeptide, via adeno-associated virus,4-6 lead to an increase in muscle mass in treated animals, with the greatest increase in FS-treated animals.7 Use of potentially therapeutic antimyostatin-blocking antibodies of high-binding affinity has proved to be a promising strategy. However, there are some constraints related to the use of antimyostatin antibodies that include difficulty in long-term sustainability, undesirable immune responses, and inhibitory effects not precisely specific to myostatin in regard to muscle growth.8,9 Significant increase in skeletal muscle mass was also observed using adeno-associated virus vectors to deliver a recombinant myostatin propeptide gene fragment, or by a retrovirus-based RNA interference system (RNAi).4,6,10 Both approaches have safety concerns of possible genotoxicity, due to uncontrolled vector genome insertion into host chromosomes.11 The RNAi system faces an additional hurdle in terms of effective delivery of the RNAi molecules into the disease models for clinical studies.12 RNA-based modulation therapy has the potential to overcome difficulties encountered by conventional gene therapy methods. Antisense oligonucleotides (AOs) are capable of hybridizing to a sense target sequence leading to cleavage of the RNA:DNA hybrid by RNase H which results in downregulation of gene transcription.13,14 In an alternative approach, antisense-mediated modulation of pre-mRNA splicing has been pioneered by Dominski and Kole.15 In the first experiments, AOs were aimed at activated cryptic splice sites in the β-globin (HBB) and cystic fibrosis transmembrane conductance regulator (CFTR) genes in order to restore normal splicing in β-thalassemia and cystic fibrosis patients.15-17 The identification of exon/intron boundaries by the splicing machinery, and therefore inclusion of the exons into the mRNA, is extensively thought to depend on exonic splicing enhancer (ESE) motifs.18 By masking these ESE sites with sequence-optimized AOs, the targeted exons are no longer recognized as exons, and are spliced out with neighbouring introns. This so-called antisense-induced exon skipping has already been used clinically to partly correct the mutated dystrophin and convert the severe Duchenne muscular dystrophy phenotype into a milder Becker muscular dystrophy phenotype.19 Clinical trials to determine the safety profile and the efficacy of single intramuscular doses of two different chemistries of AOs, 2′-O-methyl phosphorothioate (2′OMePS) AOs and phosphorodiamidate morpholino oligomers (PMOs) in Duchenne muscular dystrophy patients have recently been completed.20,21 The treatments were well tolerated by all the patients and the injection of AOs induced the production of dystrophin. 2′OMePS AOs, being negatively charged, are easily delivered in vitro, whereas PMOs are capable of more sustained effect in vivo due to their resistance to enzymatic degradation22 and owing to their longer sequence, have increased affinity to target.23 When conjugated with various peptide derivatives, or with dendrimeric octa-guanidine (so-called Vivo-morpholino), PMOs demonstrate a significantly increased delivery in the case of dystrophin skipping.24,25 